1. Field of the Invention
The present invention generally relates to a mobile terminal that controls transmission power. In particular, the present invention relates to an apparatus and method for controlling transmission power.
2. Description of the Related Art
Generally, in a wireless communication device such as a terminal and a base station used in mobile communication systems, such as a Code Division Multiple Access (CDMA) system and a Personal Communication Service (PCS) system, a mobile terminal includes a Drive Amplifier (DA) and a Power Amplifier (PA). The DA amplifies a signal, i.e., a transmission Radio Frequency (RF) signal, to a power level that is high enough for the RF signal to be power-amplified by the PA. The PA power-amplifies the RF signal output from the DA to a power level that is high enough for the RF signal to be transmitted via an antenna. The power-amplified RF signal passes through a duplexer and is transmitted wirelessly via an antenna. For example, the DA amplifies the RF signal up to about 0 dBm in order to allow the PA to amplify the RF signal. The PA then amplifies the RF signal output from the DA up to about 28 dBm and applies the RF signal to the duplexer.
In mobile communication systems such as a Code Division Multiple Access (CDMA) system and a Personal Communication Service (PCS) system, a mobile terminal divides a transmission power level into several classes within a specified level range and controls the transmission power level of one of the classes according to a propagation environment, thereby reducing unnecessary current consumption or interference. Generally, in a terminal or a base station of a CDMA or PCS system, a transmission power level is reduced with an increase in the Received Signal Strength Indicator (RSSI) of a received RF signal, and the transmission power level is increased with a reduction in the RSSI of the received RF signal.
Such a transmission power control of the mobile terminal is of high importance in the communication system. In a CDMA system, the RSSI of a received signal has to be determined, and a transmission signal has to be transmitted at a transmission power level corresponding to the determined RSSI. After a call connection with a base station, the base station may finely control the transmission power level. At this time, the mobile terminal needs to output a transmission signal at the transmission power level as controlled by the base station. As such, the transmission power control of the mobile terminal is of significant importance.
In the transmission power control, it is necessary to match the power level of a transmission signal that has to be transmitted by the mobile terminal with the power level of an output signal actually transmitted by the mobile terminal. However, due to the non-linear property of a transmitter Power Amplifier (PA), the mobile terminal stores an RF analog Subsystem (RAS) compensation table for storing a compensation value in order to output a desired transmission power level, instead of an actually output transmission power level. For the RAS compensation table, 16 power indices for a specific number of terminals are created, and Automatic Gain Control (AGC) values corresponding to the power indices are measured and stored. The RAS is used to compensate the AGC characteristic of the RF amplifier to at least improve linearity. An average value and a weight value are calculated for the stored AGC values. The RAS compensation table is a sample table that is created based on the calculated average value and weight value. For the remaining terminals, only AGC values corresponding to two power indices are measured and AGC values corresponding to the other indices are automatically stored according to weight values acquired from the RAS compensation table.
In the transmission power control, the transmission power of the mobile terminal may also vary with the temperature and the frequency of the channels. To prevent a variation in the transmission power, the mobile terminal has a temperature and channel compensation table that stores offset values to compensate for to the temperature and the frequency variation of the channels.
To control the transmission power using the RAS compensation table and the temperature and channel compensation table, the mobile terminal may be configured as illustrated in FIG. 1.
Once a reception signal received from a system is detected by an RSSI detection unit 103, a controller 100 determines a transmission power level according to the detected RSSI value and determines an AGC value corresponding to the determined transmission power level by referring to an RAS compensation table 105. The controller 100 then controls a Drive Amplifier (DA) 102 and a Power Amplifier (PA) 104 according to the determined AGC value. Under the control of the controller 100 that outputs a control signal by referring to the RAS compensation table and temperature and channel compensation table 105, a transmission signal is power-amplified by the DA 102 and the PA 104 and then transmitted via an antenna ANT.
For the transmission power control, it is necessary to compensate for the difference between the power level of the signal to be transmitted and the actually output signal power level using an RAS compensation table for storing a compensation value for compensation for the non-linear property of the PA and a temperature and channel compensation table for storing an offset value for compensation for variation with temperature and frequency channels.
However, even if a variation in the transmission power level is compensated for using the RAS compensation table, the compensation may not be accurate for every terminal because of the use of a statistical method.
Moreover, when a variation in the transmission power level is compensated for using the temperature and channel compensation table that stores offset values according to the temperature and the frequency of the channels, the compensation may not be accurate for every terminal because measurements acquired by sampling a random power variation are applied to every terminal even though a specific power variation corresponding to the temperature and the frequency of a channel exists for each terminal.